Cool gas and dust in M33: Results from the Herschel M33 extended survey (HERM33ES)

TL;DR

This study uses Herschel space telescope data to analyze cool dust and molecular clouds in M33, revealing variations in dust properties and gas content, and highlighting the importance of complete CO mapping for accurate molecular gas estimates.

Contribution

First detailed far-IR-submm observations of M33's dust and molecular clouds, linking dust emission to gas content and emphasizing the need for comprehensive CO mapping.

Findings

Dust cross-section varies across M33, peaking south of the nucleus.

H2/HI mass ratio decreases significantly from the center outward.

Morphology of gas derived from dust correlates with CO observations.

Abstract

We present an analysis of the first space-based far-IR-submm observations of M 33, which measure the emission from the cool dust and resolve the giant molecular cloud complexes. With roughly half-solar abundances, M33 is a first step towards young low-metallicity galaxies where the submm may be able to provide an alternative to CO mapping to measure their H$_2$ content. In this Letter, we measure the dust emission cross-section $\sigma$ using SPIRE and recent CO and \HI\ observations; a variation in $\sigma$ is present from a near-solar neighborhood cross-section to about half-solar with the maximum being south of the nucleus. Calculating the total H column density from the measured dust temperature and cross-section, and then subtracting the \HI\ column, yields a morphology similar to that observed in CO. The H$_2$/\HI\ mass ratio decreases from about unity to well below 10% and is about 15% averaged over the optical disk. The single most important observation to reduce the potentially large systematic errors is to complete the CO mapping of M 33.